Set gypsum (calcium sulfate dihydrate) is a well-known material used to make many types of products and included commonly in many types of products.
Land plaster is a type of gypsum which is any mixture containing more than 50% calcium sulfate dihydrate, CaSO4.2H2O (by weight) from any source (such as gypsum mineral from ground or unground sources, synthetic gypsum from flu-gas desulfurization process, or other chemical gypsum as by-products of the phosphate/fluoride industry).
Generally, gypsum-containing products are prepared by forming a mixture of calcined gypsum (i.e., calcium sulfate hemihydrate and/or calcium sulfate anhydrite) and water, and, optionally, other components, as desired. The mixture typically is cast into a pre-determined shape or onto the surface of a substrate. The calcined gypsum reacts with the water to form a matrix of crystalline hydrated gypsum, i.e., calcium sulfate dihydrate. It is the desired hydration of calcined gypsum that enables the formation of an interlocking matrix of set gypsum, thereby imparting strength to the gypsum structure in the gypsum-containing product.
U.S. Pat. No. 3,410,655 (Ruter et al.), incorporated herein by reference, teaches a process for producing alpha-calcium sulfate hemihydrate in the form of compact, non-needle-like crystals. The process elutrates calcium sulfate dehydrate with water to remove organic impurities and fine and slurry crystal portions from the calcium sulfate dehydrate at a temperature between 60 and 90 degrees C. This forms an aqueous suspension of the calcium sulfate dihydrate having a pH between about 1.5 and about 6. This also adds tetrapropylenebenzene sulfonate or carboxymethyl cellulose as a crystal form improving agent to the dehydrate suspension. After adding the agent, the process further raises the temperature of the suspension at a rate which increases with the pH of the suspension and maintains the same with close temperature control (plus or minus 1 degree C.) for a period of time to complete formation of an alpha-calcium sulfate hemihydrate.
U.S. Pat. No. 5,746,822 (Espinoza et al.), incorporated herein by reference, discloses producing a cementitious composition of the setting type which can be kept in a ready-mixed state (premixed with water) and which can be caused to set and harden upon introduction of an accelerator. As a component of such cementitious composition is mentioned calcium sulfate hemihydrate as a principle filler material which may contain certain non-calcium bearing phosphate additives which prevent setting action in the presence of water.
U.S. Pat. No. 6,228,163 (Espinoza et al. II) teaches the mixing of a dry α-calcium sulfate hemihydrate with, among other components, TSPP as a set preventer.
U.S. Pat. No. 6,409,825 (Yu et al.), incorporated herein by reference, discloses combining a wet gypsum accelerator with water and calcined gypsum to form an aqueous mixture in which the calcined gypsum is hydrated to form an interlocking matrix of set gypsum.
U.S. Pat. No. 6,805,741 (Liu et al.), incorporated herein by reference, discloses forming a ready-mixed composition or, alternatively a dry composition, which sets when mixed with an activator for the reaction of calcined gypsum and water to form set gypsum. The patent discloses to add a polyacrylic acid and/or a salt thereof as a set preventer.
There is a need for ready-mixed compositions which have lower viscosity and higher strength than ready-mixed joint compounds. For example, in many applications, it would be desirable to have a ready-mixed composition that is flowable before activator is added to the composition, and which remains flowable for some period of time even after activator is added, so that the user has sufficient “workable” time to use the composition for its intended purpose.
Flowable ready-mixed compositions would be desirable for use in a variety of interior and exterior applications. For example, flowable ready-mixed compositions would be useful in a number of plaster applications including dental applications, building plaster, machinable plaster, and the like. Also, dental plaster is useful for casting dimensionally accurate reproductions of a portion of an oral cavity or extraoral facial structure as a positive impression formed from a polymer such as alginate that forms a negative impression. In addition, building plaster may be in the form of, for example, veneer plaster (e.g., DIAMOND® brand interior finish plaster, commercially available from the United States Gypsum Company) for finishing interior wallboards.
Also, flowable ready-mixed compositions have use in forming molds made of plaster for casting pottery figurines and the like using pressure casting or slip casting techniques known in the art. Also, ready-mixed flowable compositions would be useful for solid casting applications, such as, for example, statuary and architectural pieces such as moldings, and rotational casting applications. For example, rotational casting is used to form hollow products, such as, for example, lamp posts and the like. Also, flowable ready-mixed compositions would be useful in spray applications, including glass fiber reinforced gypsum compositions for making decorative architectural pieces.
Thus, there is a need for ready-mixed, setting-type compositions having a relatively low viscosity to be flowable, and preferably remaining flowable for a time even after activator is added. Also, there is a need for such a ready-mixed composition in which setting is avoided in the absence of activator for a relatively long time such that the composition has a long shelf-life. Furthermore, there is a need for a ready-mixed composition in which the solid particulate in the composition does not settle and where water does not bleed.